Polyurethane compositions, in particular one-component polyurethane compositions, nowadays find use in various sealing and bonding applications due to their versatile mechanical and adhesion properties. They are especially suitable for elastic sealing and bonding, because, depending on the formulation, they can combine high adhesion strength with toughness and elasticity. Generally, such one-component polyurethane compositions are applied in the form of a viscous liquid or paste that cures to a soft elastic, rubberlike or tough solid, mostly by influence of humidity from air. While it is necessary for most polyurethane compositions to have a sufficiently long open time for convenient application, for certain adhesive applications it is required that the adhesive is able to hold a certain mechanical load immediately after the adhesive is applied. This is the case for example in automotive windshield repair applications, also known as aftermarket glass replacement (AGR), where the adhesive has to hold the new windshield in place immediately, while maybe allowing for some positional adjustments, before the adhesive has chemically cured. Adhesives with the ability to hold a certain load before curing are typically described as having high green strength.
Adhesive compositions with high green strength are, for example, so-called warm melt adhesives (“warm melts”), which have a highly viscous paste-like to near-solid consistency at room temperature and which for application are heated, typically to a temperature in the range from 50° C. to 100° C. The heating leads to a significant decrease in viscosity during application. The high green strength of such an adhesive stems primarily from a sharp increase in viscosity upon cooling, as result of the physical solidification of one constituent of the adhesive, known as the melting component. This melting component represents a substance which is solid at room temperature but melts when the adhesive is heated to the application temperature, and which solidifies again within a certain time, by crystallization, for example, when the adhesive is again cooled.
For example, warm melt adhesives based one-component polyurethane compositions are disclosed in U.S. Pat. No. 5,367,036. The composition described therein comprises alongside a reactive, curable polyurethane polymer a melting component in the form of a nonreactive polyurethane polymer whose isocyanate groups have been reacted with a monomer alcohol. The use of a nonreactive polyurethane polymer as melting component, however, has the disadvantage that, when the composition is cured chemically by means of moisture, the non-reactive polyurethane polymer is not chemically incorporated into the polyurethane matrix. The melting component is therefore able to migrate from the cured composition and so give rise to unwanted effects on the surface, or to lead to poorer chemical resistance, lower mechanical strength and/or poorer adhesion properties on the part of the cured composition.
Disadvantages of known polyurethane warm melts comprising reactive melting components for improved green strength on the other hand typically include their limited storage stability and their possibly premature crosslinking, thereby adversely affecting their viscosity and their solidification behavior. In general, one-part warm melt polyurethane adhesives with high green strength nowadays still suffer from short open times, as they tend to solidify too quickly after application, or they have poor storage stability or other adverse properties as described above.
Thus, there is still a need for a one-part polyurethane composition suitable for warm melt applications that combines high green strength with long open time and excellent mechanical properties and which does not suffer from poor storage stability or adverse effects from incompatible melt components.